This invention relates generally to the insertion of vertical prefabricated drains into the earth, and more specifically to a method and apparatus for remotely severing such drains after installation under a body of water.
One well known technique for improving soft, saturated soil, such as saturated clay for example, is to drive into the soil a drainage element (a prefabricated vertical drain or PV drain) that penetrates deep into the soil with the top end of the drainage element maintained above the surface of the soil. The PV drain is formed of a suitable material which is water permeable so the water in the soil can penetrate the walls of the drain and flow upwardly therein, to the surface of the soil as a result of water pressures in the soil beneath the surface. It is common practice in such situations to increase the inherent water pressures in the soil by placing a layer of earth on top of the wet soil so that the weight thereof will assist in forcing the water into and upwardly through the PV drains, where it can be readily disbursed.
The PV drains are composite drains composed of an extruded plastic core shaped to provide drainage channels when this core is wrapped in a special filter fabric generally referred to as geofabric. The geofabric is a filter fabric constructed with opening sizes such as to prevent the entrance of soil particles, but allow pore water to enter freely. The finished drain material is band-shaped, is about ⅛ to ¼ inches thick, and approximately 4 inches wide. It is provided in 4 to 5-foot diameter rolls containing 800 to 1000 feet of drain. An example manufacturer of PV drains is Nilex Construction, LLC of Centennial, Colo., U.S.A. Its product is sold under the trademark MEBRADRAIN.
Installation is accomplished by means of specialized equipment, consisting of a crane (or excavator) mounted with a mast housing a special installation mandrel. The mandrel, containing the drain, is intruded directly into the ground from the bottom of the mast. After reaching the desired depth, the mandrel is withdrawn back into the mast, leaving the undamaged drain in place within the soil. A typical installation rig may utilize roller chains to drive the mandrel, however there are a number of methods of driving the mandrel, including cables activated by rotary winches as well as linear hydraulic rams. Some units make use of rack and pinion arrangements where the rack portion is attached to the mandrel and the drive (or pinion) linkage is at the bottom of the mast. A vibratory hammer is sometimes attached to the top of the mandrel to aid in penetrating stiff or hard layers within the soil. By way of example see U.S. Pat. No. 5,213,449 for Apparatus for Inserting Wick Drains into the Earth.
FIG. 1 illustrates one typical anchor plate configuration 1. After the drain 10 is attached to the bail or handle 11 of the anchor plate 12 as indicated, the drain is pulled back manually by back spooling onto the PV drain reel, so that the anchor plate completely and firmly covers the bottom end 13 of the mandrel 14. This prevents soft soil from entering the mandrel as it is penetrated into the earth. It then acts as an anchor, holding the drain in place as the mandrel is withdrawn.
After the mandrel is withdrawn, the drain between the bottom of the mandrel and the ground is manually cut, another anchor plate is attached to the drain and the drain pulled back to again seat the anchor plate over the mandrel bottom. The rig is then moved to the next drain location, and the process is repeated.
When constructing marine earthworks (breakwaters, jetties, cofferdams, etc.) it is often desirable to install PV drains into the soil below the body of water to accelerate the consolidation of underlying soft, compressible soil on which the works are to be built. After the drains are installed it is usual practice to place a layer of stone over the drains and then build the fill or work over the stone. The stone acts as a drainage medium, allowing pore water exiting the drains to find a free drainage path from under the fill.
In these cases PV drains are often installed from a barge as illustrated in FIG. 2. The sequence of installation is essentially the same as a land operation. FIG. 2 illustrates the condition where the drain 10 has just been installed and the mandrel has been withdrawn to above water level. It would be desirable to cut the drain near to the sea bottom, but since this operation may take place in water depths up to 60 feet this is problematic.
Present practice is to cut the drains above the water level, and either leave the resulting excess drain or to weight the top end of the drain and let it sink to the bottom. In either case much drain material is wasted, and the excess drain left in the water poses a nuisance, if not a hazard. If it is essential that the excess drain material be removed it would require divers to cut the drains to length after installation.
The present invention discloses a novel method and apparatus to cut the drains near to the bottom of the body of water while working entirely from the barge.